How to introduce a puppy to a 3 year old dog who might be aggressive?

[Nikki]

So im thinking about getting a puppy soon (in a few months) but my brother has a dog already and she has been an only dog since he got her 3 years ago. His dog is a shiba inu and When i read about that breed it says they can be aggressive but I dont know if she is aggressive or not since she has never been around another dog (that i know of). A few months ago i introduced her to my friends puppy but we only let her sniff it and she SEEMED fine but then again my friend only stayed for 5 minutes. My brothers dog can be aggressive with my cats though if she is eating or has a bone.

My question is, how can i properly introduce my new puppy to my brothers dog when i dont know if his dog is aggressive or not?
I cant use treats with the older dog, she gets VERY defensive whenever treats come out. When i try giving her treats after shes nice to my cats she violently attacks them when they make a single movement. If no food or treats/bones are around she is just fine with the cats and even plays with them.
Like even if i give her just one treat and theres no sign of anymore treats she is aggressive because she thinks shes going to get more until after awhile when she knows shes not getting anymore.